The present invention relates to a novel conductive polymer composition having positive temperature coefficient (PTC) characteristics and a PTC element using the composition.
A PTC element comprising a conductive polymer which exhibits low resistivity at ambient (room) temperature and increases its resistance with increasing temperature (has a positive temperature coefficient of electrical resistance) is used as a protective element against overcurrent in batteries, electronic devices, etc. in case of abnormality.
Conventional conductive polymers for PTC elements include a composition comprising a mixture of polyethylene having a melting point of 100xc2x0 C. or higher and a copolymer having a melting point of lower than 100xc2x0 C. and a metal boride disclosed in JP-A-2000-21605 and compositions comprising a crystalline polymer such as high-density polyethylene and a conducive filler such as carbon black disclosed, e.g., in JP-A-11-214203 and JP-A-11-5915.
U.S. Pat. Nos. 5,837,164, 5,985,182, 6,074,576, and 6,090,313 (corresponding to JP-A-10-116703, JP-A-11-329076, and JP-A-2000-188206) disclose compositions comprising a semi-crystalline polymer containing nylon 11 and a carbon-based particulate conductive filler, describing that the compositions are characterized by exhibiting at least 103 times as much electrical specific resistance at a switching temperature ranging from 140 to 200xc2x0 C. as that at 25xc2x0 C.
JP-A-5-3103 discloses a radiation-crosslinked conductive polymer composition, etc.
U.S. Pat. No. 4,980,541 (corresponding to JP-T-4-500745 and JP-A-11-144907) discloses a conductive polymer composition (i) which exhibits PTC behavior, (ii) which has a resistivity Rcp of 0.01 to 100 xcexa9xc2x7cm at 20xc2x0 C., and (iii) and which comprises (a) an organic polymer having a crystallinity of at least 5% and a melting point Tm and (b) carbon black having a pH of less than 4.0, reciting olefin polymers or copolymers such as polyethylene, fluorine-containing polymers, and the like as examples of the organic polymer (a). The U.S. Patent mentions that use of carbon black having a pH of 4.0 or higher results in impairment of stability of the composition after high temperature aging.
U.S. Pat. Nos. 5,181,006 and 5,093,036 (corresponding to JP-T-4-500694) discloses a polymer thick film ink comprising (1) an organic polymer having a crystallinity of at least 5%, (2) an active solvent suitable for dissolving the polymer at room temperature, and (3) carbon black having a pH of less than 4.0. The recited organic polymer (1) includes polymers comprising at least one olefin; copolymers comprising at least one olefin and at least one monomer copolymerizable therewith, e.g., ethylene/acrylic acid, ethylene/ethyl acrylate, and ethylene/vinyl acetate; polyalkenamers such as polyoctenamer; melt-shapeable fluoropolymers, such as polyvinylidene fluoride and copolymers thereof; and blends of two or more such crystalline polymers.
U.S. Pat. No. 4,304,987 discloses a composition having PTC characteristics which comprises a crystalline polymer and carbon black, reciting olefin polymers or copolymers such as polyethylene and fluorine-containing polymers as the crystalline polymer. The U.S. Patent teaches that the composition is preferably crosslinked by irradiation or heating. As an embodiment of the PTC characteristics of the composition, the resistivity reaches the maximum at around 130xc2x0 C. and decreases in a higher temperature range.
The following reports suggest the factors by which these conventional polymer compositions exhibit PTC characteristics. Polymer, vol. 41, p. 7279 (2000) reports that the strong PTC effect of carbon black-loaded crystalline polymer compositions is attributed to an increase of average distance among carbon black particles which accompanies thermal expansion of melting polymer crystals. In Polymer Engineering and Science, vol. 39, p. 1207, (Jul., 1999), it is reported that transition from low-resistant state to high-resistant state is caused by volumetric expansion of melting polymer crystals.
In short, melting of crystalline polymers is a factor of PTC characteristics. Therefore, in application to PTC elements, the conventional polymer compositions require such steps as crosslinking in order to retain the shape of the element.
It is said that the conventional polymer compositions manifest their PTC characteristics through the following mechanism. At temperatures lower than the crystal melting point of a polymeric component of the composition, conductive particles dispersed in the noncrystalline region of the polymeric component are in contact with each other to form a route for electric conduction to show low resistance. With a rise in temperature, the crystalline region of the polymeric component melts to increase the volume of the noncrystalline region. As a result, the distance among the conductive particles increases to cut the conduction route thereby showing high resistance. On cooling down to ambient temperature, the volume of the noncrystalline region of the polymeric component reduces to bring the conductive particles closer to each other to re-form the conduction route thereby exhibiting low resistance.
In this way the conventional polymer compositions rely on melting of crystalline polymers for the PTC behavior. Therefore, in order for the conventional polymer compositions to be applied to PTC elements, they need a special process for flow prevention, such as irradiation crosslinking, so as to retain the shape of the element. Further, because the above-described polymers are slow in melting, which can result in retarded PTC behavior, sharper behavior has been desired.
Furthermore, PTC elements in some applications are required to work at lower temperatures than conventional switching temperatures, and improvement in this respect has been demanded.
Besides, PTC elements comprising some conventional polymer compositions have poor stability to repetition, and improvement in this point has also been required.
In addition, some conventional polymer compositions can exhibit negative temperature coefficient (NTC) characteristics in temperatures higher than the temperature at which resistance steeply rises because of the PTC characteristics. Because the temperature showing the NTC characteristics is close to that showing the PTC characteristics, improvement has been desired.
It is known that trans-1,4-polybutadiene has crystal polymorphism and shows phase transition at around 50 to 80xc2x0 C. That is, it is said that the crystal form in a low-temperature phase is a monoclinic phase, which changes to a pseudohexagonal crystal form at a high temperature (see, for example, Polymer Preprints, Japan, vol. 49, No. 8 (2000)).
Polymer Handbook (3rd ed.) teaches that trans-polybutadiene exhibits crystalline modifications I and II having a density of 0.97 gcmxe2x88x923 and 0.93 gcmxe2x88x923, respectively, and transition from modification I to modification II occurs at 75xc2x0 C.
Accordingly, an object of the present invention is to provide a conductive polymer composition which exhibits PTC behavior at relatively low temperature, shows sharp PTC behavior with stability to repetition, does not flow with temperature elevation and therefore needs no processing step for retaining the shape of a PTC element, and has a wide temperature latitude for producing high resistance.
The present invention accomplishes the above object by providing a conductive polymer composition exhibiting PTC characteristics and comprising 100 parts by weight of a crystalline polymer (A) and 5 to 150 parts by weight of a conductive powder (B) dispersed in the crystalline polymer (A), which is characterized in that the crystalline polymer (A) shows crystal transition.
The present invention provides the above-described conductive polymer composition characterized in that the crystalline polymer (A) increases in volume due to crystal transition thereby imparting PTC characteristics to the composition.
The present invention also provides the above-described conductive polymer composition characterized in that the crystalline polymer (A) is trans-1,4-polybutadiene having a trans-1,4-bond content of 85% or more
The present invention also provides the above-described conductive polymer composition characterized in that the conductive powder (B) is carbon black.
The present invention also provides the above-described conductive polymer composition characterized in that the carbon black has a pH of 6 or higher.
The present invention also provides a PTC element characterized by using the above-described conductive polymer composition.